Streaming content, for example video data, audio data or similar kinds of multimedia data, may be delivered on the basis of point-to-point (PTP) bearers, such as IP unicast or the streaming standard PSS (Package Switched Streaming Service) of the 3GPP (3rd Generation Partnership Project) for mobile networks. For applications such as mobile TV services the PTP distribution comprises a potentially large number of parallel PTP connections. Upcoming technologies such as the 3GPP MBMS (Multicast/Broadcast Multimedia Service) feature or DVB-H (Digital Video Broadcasting-Handhelds) enable the delivery of content streams also over point-to-multipoint (PTM) bearers in mobile networks.
Having both mechanisms, PTP and PTM distributions, available for streaming content delivery, resource usage in mobile networks can be optimised. This is because in case a streaming content service is distributed via PTP, network resources have to be allocated only for those users actually requesting the content, thus the allocated network resources are determined by the number of concurrent users and not by the number of channels of, e.g., a mobile TV service. Content delivery via a PTM distribution, on the other hand, means that the amount of allocated resources is determined by the number of channels instead of by the number of users.
In order to maximize the number of users which can be concurrently provided with streaming content for given transmission resources over the radio interface, PTP and PTM transport mechanisms may be used in combination. For example, a popular content may be distributed via a broadcast bearer, while a less popular content may be distributed via unicast connections.
The number of users interested in a particular streaming content generally changes with time. For example, while many users may be interested in a particular TV channel in the daytime, only few users may watch this channel at night. A particular program may encounter a large interest, for example a popular N-series, while the next program on this channel may be less popular. Therefore, it is desirable to be able to switch the distribution mode for a particular content stream from PTP to PTM and vice versa. Preferably the switching should occur seamless, i.e. the presentation of the streaming content on the terminal device should not encounter any gaps, freezing or repetitions. However, while the distribution mode may be seamlessly switched in a straightforward way from PTM to PTP, switching from PTP to PTM poses problems in this respect.
In a terminal device, for example a mobile terminal having implemented a video client adapted for the reception of mobile TV, typically a buffer is provided for buffering the received streaming content. This avoids pauses or gaps in the presentation of the content to the user which may otherwise occur due to delays in the delivery of the content to the terminal. Due to limited storage resources in the mobile terminal, a buffer typically has a maximum allocatable size which is sufficient to temporarily store, e.g., few seconds of a video stream. Consider a plurality of terminal devices each receiving one and the same streaming content via individual PTP bearers. In general, the content buffers in the terminals will be in a different status between the terminals because of different individual delays resulting from connection setup, channel switching, the different connection qualities, etc. Therefore, neither the play-out point, i.e. the content buffer fragment (e.g. video frame or frames) scheduled at each particular moment for presentation, nor the begin-of-buffer, i.e. the most recently received fragment of the content will be identical from buffer to buffer.
For seamless switching of content delivery from PTP to PTM the stored content in the buffers has to overlap sufficiently to compensate for gaps occurring in the transmission due to the switching. However, the differences in buffer status in combination with the limited (maximum) buffer size may in many cases prevent a seamless switching. For example, in a first buffer the play-out point may well be located at a later time than the begin-of-buffer in a second buffer, i.e. there is no overlap at all between these buffers. Setting the play-out time of the content stream in the PTM distribution to the play-out time of the first buffer (or a later timepoint) would lead to a gap in the presentation on the terminal with the second buffer. Setting the play-out time of the PTM distribution to any earlier timepoint, e.g. the begin-of-buffer of the second buffer, would lead to a gap in or a freezing of the presentation in the terminal with the first buffer, because the terminal will play out the first buffer content and then has to wait until new, un-presented content will be available from the PTM distribution.
Another known solution, proposed for example in the IETF (Internet Engineering Task Force) RFC (Request For Comments) draft “Multiple aggregated control URIs for RTSP” (available as textfile ‘draft-einarsson-mmusic-rtsp-macuri-01.txt’ from www.ietf.org), is to deliver the streaming content for a certain time period in parallel via a plurality of unicast connections and via broadcast. This technique has the drawback of requiring considerable resources in the distributing network during parallel delivery, which is particularly relevant in mobile networks because of their generally limited transmission capacities over the radio interface.